Influence of liquid layers on energy absorption during particle impact

The influence of the thickness of a covering liquid layer and its viscosity as well as the impact velocity on energy loss during the normal impact on a flat steel wall of spherical granules with a liquid layer was studied. Free-fall experiments were performed to obtain the restitution coefficient of elastic–plastic Al2O3 granules by impact on the liquid layer, using aqueous solutions of hydroxypropyl methylcellulose with different concentrations for variation of viscosity (1–300 mPa s). In the presence of a...     

Experimental study of shock-accelerated liquid layers

An experimental investigation of a shock wave interacting with one, or several, liquid layer(s) is reported with a motivation towards first wall protection in inertial fusion energy reactor chamber design. A 12.8 mm or 6.4 mm thick water layer is suspended horizontally in a large vertical shock tube in atmospheric pressure argon and subjected to a planar shock wave of strength ranging from M = 1.34 to 3.20. For the single water layer experiments, the shock-accelerated liquid results in a significant increase in end-wall pressure loading (and impulse) compared with tests without water. The end-wall loading can be reduced by more than 50% for a given volume of water when it is divided into more than one layer with interspersed layer(s) of argon. A flash X-ray technique is employed to measure the volume fraction of the shocked water layer and multiple water layers are found to dissipate more energy through the liquid fragmentation process resulting in increased shock mitigation.     

Effect of particle polydispersity on micromechanical properties and energy dissipation in granular mixtures

A series of numerical tests was conducted to study the micromechanical properties and energy dissipation in polydisperse assemblies of spherical particles subjected to uniaxial compression. In general, distributed particle size assemblies with standard deviations ranging from 0% to 80% of the particle mean diameter were examined. The microscale analyses included the trace of the fabric tensor, magnitude and orien- tation of the contact forces, trace of stress, number of contacts and degree of mobilization of friction in contacts between particles. In polydisperse samples, the average coordination numbers were lower than in monodisperse assemblies, and the mobilization of friction was higher than in monodisperse assemblies due to the non-uniform spatial rearrangement of spheres in the samples and the smaller displacements of the particles. The effect of particle size heterogeneity on both the energy density and energy dissipation in systems was also investigated.     

An experimental study for impact of a drop onto a particle in mid-air: The influence of particle wettability

Drop impact onto a particle in mid-air is a fundamental physical phenomena relevant to many applications involving a fluidized bed, for example, refining of heavy oil or coating of drug particles for pharmaceutical industry. A literature analysis shows that previous studies were focused on collisions of drops with static particles of either spherical or non-spherical shapes. This paper reports on experiments conducted at similar impact conditions with water drops and particles made of hydrophobic (polystyrene) and hydrophilic (soda-lime glass) materials. It was observed that at similar Weber numbers, wettability of the particle plays a key role in transition between collision outcomes with formation of a liquid lamella and ligaments. Present investigation is one of the first which reveals quantitative aspects for dynamics of the lamella and liquid ligaments in a particular case of mid-air collisions.     

Effect of particle polydispersity on micromechanical properties and energy dissipation in granular mixtures

A series of numerical tests was conducted to study the micromechanical properties and energy dissipation in polydisperse assemblies of spherical particles subjected to uniaxial compression. In general, distributed particle size assemblies with standard deviations ranging from 0% to 80% of the particle mean diameter were examined. The microscale analyses included the trace of the fabric tensor, magnitude and orientation of the contact forces, trace of stress, number of contacts and degree of mobilization of friction in contacts between particles. In polydisperse samples, the average coordination numbers were lower than in monodisperse assemblies, and the mobilization of friction was higher than in monodisperse assemblies due to the non-uniform spatial rearrangement of spheres in the samples and the smaller displacements of the particles. The effect of particle size heterogeneity on both the energy density and energy dissipation in systems was also investigated.     

粒度对HMX撞击感度的影响

采用12型工具法对炸药粒度与HMX(Cyclotetramethylenetetranitramine,奥克托今)撞击感度的关系进行了研究,并对撞击起爆机理进行了分析和探讨。结果表明:HMX炸药粒度对撞击感度有显著影响,且随着炸药粒度的增大,其撞击感度呈增大趋势。点火阶段对撞击起爆过程起决定性作用,影响点火阶段的因素就是决定其撞击感度高低的关键因素。晶体内部的活性中心是炸药受撞击时的起爆点,而且大晶粒炸药中更易形成优先点火的活性中心。     

Modelling of a single drop impact onto liquid film using particle method

The process of single liquid drop impact on thin liquid surface is numerically simulated with moving particle semi‐implicit method. The mathematical model involves gravity, viscosity and surface tension. The model is validated by the simulation of the experimental cases. It is found that the dynamic processes after impact are sensitive to the liquid pool depth and the initial drop velocity. In the cases that the initial drop velocity is low, the drop will be merged with the liquid pool and no big splash is seen. If the initial drop velocity is high enough, the dynamic process depends on the liquid depth. If the liquid film is very thin, a bowl‐shaped thin crown is formed immediately after the impact. The total crown subsequently expands outward and breaks into many tiny droplets. When the thickness of the liquid film increases, the direction of the liquid crown becomes normal to the surface and the crown propagates outward. It is also found that the radius of the crown is described by a square function of time: r_C = [c(t ? t₀)]^0.5. When the liquid film is thick enough, a crown and a deep cavity inside it are formed shortly after the impact. The bottom of the cavity is initially oblate and then the base grows downward to form a sharp corner and subsequently the corner moves downward. Copyright © 2004 John Wiley & Sons, Ltd.     

冲击速度和轴向静载对红砂岩破碎及能耗的影响

地下岩体工程爆破开挖中,距爆源不同距离处岩体承受的地应力和动载荷大小不同,从动载荷的角度表征岩石动态破坏结果与工程实际更吻合。为研究动载荷和地应力大小对岩体破碎和能量耗散特性的影响,利用动静组合加载试验装置,分别设置7个冲击速度和轴向静应力等级,对红砂岩试件进行冲击试验。根据试件的破碎状况,分析不同静应力工况下冲击速度对岩石破坏模式和机理的影响。计算不同工况下的应力波能量值,研究冲击速度和轴向静应力对岩石能耗特性的影响。对破坏试件进行筛分试验,研究岩石破碎分形维数随冲击速度和轴向静应力的变化关系。结果表明,随着冲击速度的增大,试件的破坏程度逐渐加大。无轴压时岩石试件破坏后整体仍是一个圆柱体,属于张拉破坏;有轴压时岩石试件宏观破坏后呈沙漏状,属于拉剪破坏。岩石耗散能随冲击速度的升高呈二次函数关系递增;轴向静应力越高,递增幅度越小。随着冲击速度的升高,岩石分形维数由零逐渐增加;随着轴向静应力的升高,分形维数由零转为大于零的临界冲击速度先升高后降低。

     

聚乙烯泡沫单填充纸瓦楞管的轴向跌落冲击缓冲吸能特性

利用聚乙烯闭孔泡沫单填充纸瓦楞管开展轴向跌落冲击试验,对比分析了结构参数和冲击参数对其缓冲吸能特性参数（比吸能、行程利用率、压缩力效率、比总体效率）的影响。结果表明,X向单填充管的动态缓冲吸能特性优于Y向单填充管,而静态缓冲吸能特性差于Y向单填充管。正四边形单填充管的动态缓冲吸能特性优于正五、六边形单填充管,X向正四边形单填充管的比吸能相较于正五、六边形管分别提高了114.4%和182.3%。对于跌落冲击压缩,单填充管的比吸能、行程利用率、比总体效率随着管长比的增大而减小,管长比为1.4的X向单填充管的比吸能相较于管长比为2.2和3.0的单填充管分别增加了45.8%和117.9%,而压缩力效率随着管长比的增大而增大。随着跌落冲击质量或冲击能量的增加,比吸能、行程利用率、压缩力效率和比总体效率皆呈增大趋势,冲击质量对X向单填充管的影响较大,而冲击速度则对Y向单填充管的影响较大。

     

Numerical study of flow and heat transfer during a high-speed micro-drop impact on thin liquid films

Numerical simulation of high-speed micro-droplet impingement on thin liquid film covering a heated solid surface has been carried out. Effect of droplet Weber number and liquid film thickness on the characteristics of flow and heat transfer has been investigated using the coupled level set and volume of fluid method. The code is validated against both the experimental and numerical results from the literature. Results show that the crown dynamics is mostly affected by variations in the initial film thickness but is weakly influenced by changes in the Weber number. The liquid within the film can be categorized as three regions based on the heat transfer distribution: the static film region, the transition region, and the impact region. The transient local wall temperature shows three stages: first stage when the temperature decreases rapidly, followed by a second stage in which the temperature starts to rise and then becomes almost constant in the third stage. After drop impact, the local Nusselt number continuously increases until reaching a maximum value, and then decreases approaching the initial impact stage. Our analysis of the change in Weber number shows that larger Weber number contributes to intense temperature variation at the crater core relative to other radial locations. Lastly, the results reveal that the thinner liquid film leads to lower wall temperature and hence, higher average Nusselt number.     

Influence of the projectile composition and shape during an impact

Slug impact is considered a potential risk for pressurized water reactors in the event of a severe accident. The German team of FZK investigates this issue through the reduced-scale experiments BERDA, whose goal is to quantify the maximum mechanical energy that the upper head of a PWR is able to withstand. As these tests are performed with a material different from the one that would intervene in a real accident, the French team of CEA studies the influence of the slug material with the SKIPPY mock-ups. In order to understand better the impact process, relevant theoretical study has been conducted with the EUROPLEXUS code Published in Prikladnaya Mekhanika, Vol. 42, No. 5, pp. 3–15, May 2006.     

基于量纲分析的泡沫铝板吸能研究

基于落锤冲击试验,以含有5个变量的泡沫铝板受冲击的能量吸收规律为研究对象,运用量纲分析法描述试验系统中各参数之间的内在联系.以落锤冲击试验为基础进行仿真试验设计,利用Abaqus软件进行仿真试验并求得相应的仿真数据,最后得到泡沫铝板受冲击后的吸能量与参数之间的方程式,检验结果显示方程式对能量吸收值具有较好的预测性.     

厚度效应对梁冲击响应的影响

用一种半解析法——间接模态叠加法,研究了质点与弹性力学梁的冲击问题,这种方法避免了具有未知奇异载荷项的平衡微分方程求解问题。由于可以用解析方法得到简支弹性力学梁的模态函数,并且能够以显式形式给出其频率方程,因此以质点与简支弹性力学梁的冲击问题为例,来考察厚度效应对瞬态响应的影响,并将所得结果与用Timoshenko梁理论所得结果进行了比较,说明了厚度效应在梁冲击问题中的重要影响。讨论了纵波和剪切波对撞击力等动力响应的影响。     

Off-centered impact of water droplets on a thin horizontal wire

The current work focuses on the initial basic stage of filtering droplets. We study the factors that have an effect upon the amount of liquid trapped on a dry smooth horizontal wire. These factors include the impact velocity, impact eccentricity, initial drop size, wire thickness, and liquid thermodynamic properties. Based on elementary analysis of some simple experiments, we propose a non-dimensional criterion for the critical eccentricity for which the maximum amount of liquid is trapped by the wire.     

多种截面形状薄壁管轴向冲击吸能特性对比研究及优化设计

利用经过台车碰撞试验验证的铝合金薄壁管材料参数对比研究了方形、六边形、八边形和圆形的多种不同截面形状薄壁管在轴向动态冲击下的吸能特性。结合多目标优化算法对吸能特性较好的八边形多胞管进行了参数优化,并提出了一种新的优化设计方案。该方案将多胞管外管、肋板和内管的厚度分别作为优化设计变量,通过与将多胞管取统一厚度作为设计变量的传统方案进行对比发现,采用该方案对八边形多胞管进行优化后明显提高了其在相同压缩力峰值下的能量吸收效率。     

基于连续-非连续单元法的三维脆性颗粒冲击破碎特性分析

通过在连续-非连续单元法(CDEM)中引入考虑应变率效应的断裂能本构以及能量统计算法,实现了球体冲击破碎过程中损伤破裂程度及能量演化的定量分析。计算结果表明,冲击破碎过程分为接触蓄能阶段、损伤破碎阶段和碎块飞散阶段。首先,颗粒的部分动能转化为单元弹性变形能,随后这部分变形能和动能迅速转化为摩擦消耗、阻尼消耗及弹簧断裂能,破碎基本完全后碎块继续飞散。不同冲击速度下,颗粒分别出现了反弹、开裂、破碎和粉碎的现象。随冲击速度的增加,D50的变化速率逐渐放缓,破碎块度逐渐趋于稳定;破裂度、损伤度以及平均损伤因子的变化速率先增加后放缓,颗粒破坏以拉伸破坏为主。以上结论可为脆性材料冲击破碎工艺的优化设计提供依据。     

Two-dimensional problem of the impact of a vertical wall on a layer of a partially aerated liquid

The two-dimensional unsteady problem of the impact of a vertical wall on a layer of a liquid which is mixed with air near the wall and does not contain air bubbles away from the wall is solved in a linear approximation. The gas-liquid mixture is modeled by a homogeneous, ideal, and weakly compressible medium with a reduced sound velocity dependent on the air concentration in the gas-liquid mixture. Outside the gas-liquid layer, the liquid is considered ideal and incompressible. During the initial stage of the impact, the liquid flow and the hydrodynamic pressure are determined using the linear theory of the potential motion of an inhomogeneous liquid. The dependence of the amplitude of the impact pressure along the wall on the air concentration in the gas-liquid layer and on the thickness of this layer is investigated. For a small relative thickness of the layer, the thin-layer approximation is used. It is shown that the solution of the original problem tends to the approximate solution as the thickness of the layer decreases. It is shown that the presence of the gas-liquid layer leads to wall pressure oscillations. Estimates are obtained for the pressure amplitude and the oscillation period. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 5, pp. 34–46, September–October, 2006.     

The role of plastic deformation on the impact behaviour of high aspect ratio aluminium foam-filled sections

The main objective of this work is to investigate the role of the plastic deformation of metal foams on the dynamic behaviour of aluminium foam-filled columns with respect to their energy absorbing capabilities. The influence of the cross-section shape as well as other parameters is thoroughly studied. A comparison with correspondent hollow-sections is performed concerning the dissipation of kinetic energy and the obtained deformed profiles. For this particular purpose, three-dimensional finite element modelling dynamic analyses are carried out using ABAQUS/Explicit in order to achieve an in-depth study of the structural crash behaviour, during which energy needs to be absorbed in a controlled manner. A comprehensive numerical study of the crush behaviour of aluminium foam-filled sections undergoing axial compressive loading is performed. The results obtained are also analysed with respect to the reduction in the length of the structural element and impact time, the effect of friction between the foam and the outer skin, the energy decomposition, the role of plastic deformation, the influence of the skin material and impact velocity, and the influence of the shape of the cross-section on the impact behaviour. A comparison with existing analytical expressions is made in order to corroborate the numerical results.     

频繁动力扰动对围压卸载中高储能岩体的动力学影响

基于冬瓜山铜矿深部巷道围岩开挖过程中面临的高应力和频繁爆破扰动问题,利用改进的SHPB动静组合加载系统,开展了频繁动力扰动对围压卸载中高储能岩体动力学影响的研究。研究结果表明,围压卸载中的矽卡岩受到动力扰动时,其动态峰值应力和弹性模量随动力扰动次数非线性变化。围压卸载中的高储能矽卡岩受到动力扰动时会释放能量。轴压促使岩样内微裂隙轴向发育,造成岩样抵抗动力扰动能力减弱;围压减缓岩样内微裂隙轴向发育,造成岩样抵抗动力扰动能力增强。动力扰动对微裂隙扩展有促进作用,使围压卸载中的岩样由拉伸破坏向剪切破坏转变。